cmd/coordinator: set appropriate css cache-control headers
cmd/coordinator: set appropriate css cache-control headers

Set headers to make browsers validate the freshness of css. Without this
change, the user must force-reload the page to pick up new changes, even
though last-modified is set correctly.

Change-Id: Ie0d65433e61965cf24e7fb15d605647fbdbdb72e
Reviewed-on: https://go-review.googlesource.com/c/build/+/419975
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Auto-Submit: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>
Run-TryBot: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>

golang/build,golang/build,golang/build,golang/build